«فَأَبْرَزْتُ دِينَارًا وَقُلْتُ لَهُ اخْتِبَارًا: إِنْ مَدَحْتَهُ نَظْمًا فَهُوَ لَكَ حَتْمًا»
I produced a dinar and challenged him: if you praise it in verse, it is certainly yours.
Abraz(tu) means producing or bringing something into view; ikhtibāran indicates testing; naẓman means in verse. In ... fa-huwa laka expresses a condition and its promised result.
A request for charity becomes a display of improvised poetic skill. The promised coin gives the stranger both subject matter and incentive.